Magnetic recording material, particularly an audio cassette tape has recently been used for recording music. As a result, the material must have a low harmonic distortion factor, excellent electromagnetic properties as well as excellent running properties and durability.
High density recording has been realized in a video cassette tape by shortening the recording wave length and narrowing the width of the tape. For high density recording, a tape must have high output, a high S/N ratio and excellent electromagnetic properties. The total thickness of the tape becomes very thin, for example 20 .mu.m or less, and a VTR tape must have far better running durability. That is, both audio cassette tapes and video cassette tapes must have much better electromagnetic properties, running properties and durabilities.
Various binder compositions have been proposed to meet the above requirements, but satisfactory binder composition has not yet been found.
Currently, a mixture of copolymers of vinyl chloride, vinyl acetate and other vinyl compounds with rubber type resins is mainly used as a binder composition in view of its dispersibility of ferromagnetic particles and running properties and durability of a magnetic layer. Copolymers of vinyl chloride, vinyl acetate and other vinyl compounds include compolymers of vinyl chloride, vinyl acetate and vinyl alcohol, and copolymers of vinyl chloride, vinyl acetate and maleic anhydride as described in, for example, U.S. Pat. Nos. 2,607,710, 2,885,365, 3,242,005, 3,247,017, 3,404,997, 3,597,273, 3,650,828, 4,238,548, 4,323,628, 4,340,644, 4,352,859, 4,388,376, 4,409,291, 4,409,299, 4,411,957 and 4,414,288. Copolymers of vinyl chloride, vinyl acetate and vinyl alcohol are more frequently used because these copolymers are very effective for dispersing ferromagnetic particles.
However, a magnetic recording medium having excellent electromagnetic properties and satisfactory running durability cannot be obtained using copolymers of vinyl chloride, vinyl acetate and vinyl alcohol.
U.S. Pat. No. 3,728,262 discloses a process in which ferromagnetic particles are treated with a diluted solution of copolymers of vinyl chloride, vinyl acetate and vinyl alcohol with stirring. However, this process does not give rise to a magnetic recording medium having excellent electromagnetic properties and satisfactory running durability.